Inflammatory bowel disease (IBD) patients frequently require thiopurine therapy to achieve remission. Thiopurines are widely used immunomodulators which have proven benefits in inflammatory bowel disease (IBD) patients who have failed 5-aminosalicylic acid (5-ASA) compound treatment. These immunomodulators can effectively induce both clinical and endoscopic remission, are steroid-sparing, and have even been shown to have benefit in fistulizing Crohn's disease. Unfortunately, thiopurines have a narrow therapeutic index, and their pharmacokinetics vary widely between individuals. Rare patients with low levels of the enzyme thiopurine methyltransferase (TPMT) are at significant risk of severe bone marrow suppression and patients with very high levels of TPMT are unlikely to respond to thiopurines. Traditionally, the balance between efficacy and risk with thiopurines has been managed by experienced physicians monitoring results of a complete blood count (CBC) and chemistry panels as well as overall clinical symptoms. These experienced, or expert, physicians use a gestalt impression of how a few of the variables in the CBC and chemistry panels should look when a patient is responding or not responding to the thiopurine treatment. However, there is no evidence that this subjective approach is effective and because this approach is dependent on expert assessment, the approach is difficult to reproduce.
Recently, a more reproducible approach to thiopurine risk management and evaluation of clinical efficacy has used the monitoring of 6-thioguanine (6-TGN) and 6-methylmercaptopurine (6-MMP) metabolites. Measurement of metabolites compensates for the individual patient variation in metabolism (pharmacokinetics) of thiopuine medications. 6-TGN is an active metabolite, and high levels correlate with good clinical response. 6-MMP is a shunt metabolite, and patients who shunt to 6-MMP are less likely to benefit from thiopurine treatment, and may actually develop liver toxicity. Specifically, 6-TGN levels greater than 230 pmol/10e8 RBCs are associated with clinical response, and 6-MMP levels greater than 5700 pmol/10e8 RBCs are associated with an increased risk of hepatoxicity. However, a recent meta-analysis of studies of these metabolites shows that their sensitivity for clinical response is only 62 percent and their specificity is only 72 percent. Additionally, monitoring these metabolites tests are costly, and slow (e.g., typical time to yield results is 5 days).
There remains a need in the art for inexpensive and accurate tests to determine probability of clinical response to thiopurine therapy, probability of patient adherence to (or conversely non-compliance with) the thiopurine treatment regimen, and probability of patient shunting of thiopurines to inactive metabolites. There is evidence that in addition to the pharmacokinetic variation between patients in their metabolism of thiopurines, there is also substantial variation in the pharmacodynamics of thiopurines between patients. This variation in the therapeutic effect of a given amount of drug or metabolite on the individual can not be predicted by metabolite testing.